The present invention relates to a lithium secondary battery which is suitably used for driving a motor of particularly an electric vehicle or the like. More particularly, the present invention relates to a lithium secondary battery in which internal resistance is reduced by clarifying a correlation between a form and the like of an internal electrode body and conditions for attaching current collecting tabs and which gives a good charge-discharge property, high output, and high current.
In recent years, while it is eagerly desired to regulate the emission of exhaust gas including carbon dioxide and other harmful substances with the elevation of environment protection campaigns as a background, in the automobile industry, in replacement of automobiles using fossil fuels, such as a vehicle driven by gasoline, the campaign to promote introduction of an electric vehicle (EV) and a hybrid electric vehicle (HEV) has become active.
A lithium secondary battery as a motor-driving battery in EV and HEV is required to have such characteristics as large battery capacity and high battery output to obtain predetermined accelerating ability, gradability and, continuous running ability. For example, in the case of HEV, since a motor is in a mode of assisting the output upon acceleration, the battery which drives the motor is required to have a high output. Therefore, a lithium secondary battery having high energy density is said to be the most preferable one as a battery for driving a motor. However, a voltage per a unit battery depends on the material forming the battery. Since a lithium secondary battery has a voltage of at most about 4.2V, a large output means a large current flow.
Since a plurality of batteries are connected in series to secure a voltage required to drive a motor, the same amount of current flows in each of the batteries. Indeed, in HEV or the like, a current of 100 A or higher often flows. In order to realize such a high output property and a high current property, it is important to reduce an internal resistance of a battery as much as possible.
In the aforementioned lithium secondary battery for HEV or the like, an electrode area in an internal electrode body is naturally large because a battery capacity per unit battery is large. Here, a current collecting tab which connects an internal electrode body with a current extracting terminal plays an important role in taking current effectively out of a battery having a large electrode area. That is, a high resistance of the current collecting tab causes a problem of high energy loss at the time of charging-discharging or melting of the tab.
It can be easily considered that the whole resistance of current correcting tabs can be reduced if the number of the current correcting tabs to be attached is increased. However, this case brings about a difficulty in an operation of attaching all the current correcting tabs to one portion collectively in a process of manufacturing a battery.
On the other hand, in an internal electrode body, a length (length in a winding-axial direction of an electroactive material layer) and a width (width of the electroactive material) of an electrode can be varied, and it is not natural that the number of current collecting tabs should be fixed in various kinds of batteries having various battery capacities. Nevertheless, influence of a relation between length or width of electrode or battery capacity and the number of current collecting tabs on an internal resistance of a battery has not been clarified.